Obviously you have to pick the MEDIC to get this. The co-op missions on NORMAL difficulty all have some difficult spots where people tend to die often. Just stick behind, pull out your defi (right on the D-PAD) and revive them.
On easy people die more rarely.
When you have defi or health pack equiped you will see a number in the bottom right corner. 100 or less. This is like the ammo of this pack. It recharges automatically.But you can only revive when it is full at 100.
For everyone that does not often play medic
: if a medic does not revive you instantly, it is most likely not because they don't like you. But they have to wait for the defi to recharge to 100.
This achievement is cummulative over multiple matches and you can check your progress unter the challenges tab.
From the main menu select PROGRESS, RB to CHALLENGES, middle column BADGES, scroll down to FIELD DOCTOR. It says xx / 100.
The icon looks like this: